A well-known method of producing silicon single crystals in particular for manufacturing semiconductor products consists in melting silicon in a receptacle, in putting a crystal germ having a desired crystal arrangement into contact with the bath of liquid silicon, so as to initiate solidification of the silicon contained in the crucible with the desired crystal arrangement, and in mechanically withdrawing an ingot of single crystal silicon obtained in this way from the crucible. This method is known as the Czochralski method or as the “CZ” method.
The receptacle containing the molten silicon is frequently a crucible made of silica or quartz (SiO2) placed in a bowl, sometimes called a susceptor, which is generally made of graphite. Heating can be provided by radiation from an electrically conductive cylindrical body made of graphite, e.g. heated by the Joule effect, which surrounds the bowl. The bottom of the bowl stands on a support. For this purpose, the bottom of the bowl is generally machined, in particular so as to form a bearing surface for centering purposes and also a support zone. In addition, in the application in question, very high purity requirements make it necessary to use raw materials that are pure, with methods that do not pollute them, and/or with methods of purification in the finished state or in an intermediate state of bowl manufacture. For carbon-containing materials (such as graphite or C/C composites), methods of purification by high temperature treatment (at more than 2000° C.) under an atmosphere that is inert or reactive (e.g. a halogen) are known and are commonly used.
The pieces of graphite used as bowls are fragile. They are often made up of as a plurality of portions (so-called “petal” architecture) and they cannot retain molten silicon in the event of the crucible made of silica leaking or rupturing. This safety problem becomes more critical with the increasing size of the silicon ingots that are drawn, and thus with the increasing mass of the liquid silicon. Furthermore, graphite bowls are generally of short lifetime while being very thick and thus bulky and heavy.
To avoid these drawbacks, proposals have already been made to make bowls out of C/C composite material. Such a material has much better mechanical strength than graphite. Making bowls of large diameter, e.g. as great as or even more than 850 millimeters (mm) can then be envisaged in order to deal with the requirement for monocrystalline silicon ingots of larger section. In addition, the thickness of such bowls can be decreased compared with the thickness of graphite bowls, thus improving the transmission of heat flux to the crucible and reducing bulk. Furthermore, C/C composite materials are less exposed than graphite to becoming brittle following corrosion from SiO coming from the crucible.
The manufacture of a C/C composite material piece, or more generally a piece of thermostructural composite material, usually comprises making a fiber preform having the same shape as the piece that is to be made, and that constitutes the fiber reinforcement of the composite material, and then densifying the preform with the matrix.
Techniques presently in use for making preforms include winding yarns by coiling yarns on a mandrel having a shape that corresponds to the shape of the preform that is to be made, draping which consists in superposing layers or plies of two-dimensional fiber fabric on a former matching the shape of the preform to be made, the superposed plies optionally being bonded together by needling or by stitching, or indeed by three-dimensional weaving or knitting.
The preform can be densified in well-known manner using a liquid process, a gas process, or a dual process combining both of them. Liquid process densification consists in impregnating the preform—or in pre-impregnating the yarns or plies making it up—with a matrix precursor, e.g. a carbon or ceramic precursor resin, and in transforming the precursor by heat treatment. Gas densification, known as chemical vapor infiltration, consists in placing the preform in an enclosure and in admitting a matrix-precursor gas into the enclosure. Conditions, in particular temperature and pressure conditions, are adjusted so as to enable the gas to diffuse into the core of the pores of the preform, so that on coming into contact with the fibers it forms a deposit of matrix-constituting material thereon by one of the components of the gas decomposing or by a reaction taking place between a plurality of components of the gas.
For pieces that are of hollow shape that cannot be developed, for example pieces that are bowl shaped, a particular difficulty lies in making a fiber preform having the right shape.
The filament-winding technique is very difficult to implement in order to obtain a bowl shape as a single piece. A solution that can be recommended is to make the side wall of the bowl preform by winding a filament and to make the portion of the preform that corresponds to the bottom of the bowl separately.
The technique of draping plies is also difficult to implement for shapes that are this complex when it is desired to avoid forming extra thickness due to folds in the plies. A known solution consists in cutting the plies, in particular to form slots, as a function of the shape that is to be made so that the plies can fit closely on this shape with the lips of the cutouts or slots coming together once draped and shaped. Such plies must be precut with very great precision. Cut plies also present the drawback of leaving discontinuities in the yarns of the preform.